From chemistry-request#* at *#server.ccl.net Tue Sep 18 06:46:15 2001 Received: from mel.ruc.dk ([130.225.220.27]) by server.ccl.net (8.11.0/8.11.0) with ESMTP id f8IAkE330320 for ; Tue, 18 Sep 2001 06:46:15 -0400 Received: from smtprelay.ruc.dk (smtprelay.ruc.dk [130.225.220.22]) by mel.ruc.dk (8.9.3/8.9.1) with ESMTP id MAA29552 for ; Tue, 18 Sep 2001 12:46:14 +0200 (MET DST) Received: from virgil.ruc.dk (virgil.ruc.dk [130.225.220.110]) by smtprelay.ruc.dk (8.9.3+Sun/8.9.3) with ESMTP id MAA15117 for ; Tue, 18 Sep 2001 12:22:49 +0200 (MEST) Received: from VIRGIL/SpoolDir by virgil.ruc.dk (Mercury 1.44); 18 Sep 01 12:46:13 +0100 Received: from SpoolDir by VIRGIL (Mercury 1.44); 18 Sep 01 12:46:11 +0100 From: "Jens Spanget-Larsen" Organization: Roskilde Universitetscenter To: chemistry \\at// ccl.net Date: Tue, 18 Sep 2001 12:45:58 +0100 Subject: CCL:Alternant hydrocarbons: "Pairing Symmetry" X-Confirm-Reading-To: "Jens Spanget-Larsen" X-pmrqc: 1 Priority: normal X-mailer: Pegasus Mail for Windows (v2.23) Message-ID: <1C01B97B4047;at;virgil.ruc.dk> CCL:Alternant hydrocarbons ... Just some additional comments: One very interesting aspect of the electronic structure of alternant hydrocarbons is the orbital and state pairing symmetry predicted by simplified model Hamiltonians (e.g., HMO and PPP). This symmetry has important consequences for the predicted electronic transitions for this class of compounds. For example, only dipole transitions between so-called plus and minus states are allowed by the pairing symmetry, and a mirror image relationsship is predicted for the spectra of paired cations and anions. There is strong experimental evidence for the presence of effective orbital and state pairing in alternant hydrocarbons, particularly in benzenoid PAHs, and there can be no doubt that the pairing theorem offers a unique insight into their electronic structure. However, perfect pairing symmetry is an abstract, purely mathematical concept, based an idealized model assumptions. In fact, the pairing properties predicted by a variety of popular quantum chemical procedures are highly sensitive to calculational details. For example, the degeneracies required by the pairing theorem are not even approximated by the CNDO/S method of Del Bene and Jaffe. It seems that a remarkable coincidence, an "accidental cancelation of terms", is responsible for the presence of near-perfect pairing symmetry in benzenoid hydrocarbons.*) Yours, Jens >--< *) J. Spanget-Larsen: "The alternant hydrocarbon pairing theorem and all-valence electrons theory. ..", Theor. Chem. Acc. 98, 137-153 (1997). =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- JENS SPANGET-LARSEN Office: +45 4674 2710 Department of Chemistry Fax: +45 4674 3011 Roskilde University (RUC) Cell-Phone: +45 2320 6246 P.O.Box 260 E-Mail: JSL -x- at -x- virgil.ruc.dk DK-4000 Roskilde, Denmark http://virgil.ruc.dk/~jsl =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-